New firmware coming
Canon has posted a article in their forum indicating that a firmware fix for the 5D Mark III and the 1dx is being tested to speed up autofocus when using the AF assist liight on a Canon Speedlight. Many users have noted that focus is slow to be achieved compared with older models. A quote from their post is given below.

“Canon has confirmed that, depending upon the shooting conditions, it may take slightly longer for the EOS 5D Mark III and EOS-1D X digital SLR cameras to acquire focus when using the Speedlite’s AF Assist Beam, compared with that of the EOS 5D Mark II and 1dx digital SLR cameras using the Speedlite’s AF Assist Beam. We are developing a firmware update to enhance the focusing feature for the EOS 5D Mark III and the EOS-1D X. The schedule for release of this firmware update will be announced shortly.”